Using a neural network for learning a Motion Graph?
... the neural network has to learn the transition probability between two keyframes. And a second neural network can then produce the motion plan which is also be trained by a large corpus. Is that idea possible or is it the wrong direction?


That would be trivial.

More difficult, but not hard, would be to take a 14 hour video of someone and create a new video with them saying something they have never said previously. Here is a demonstration, from the BBC YouTube channel, titled: "Fake Obama created using AI video tool - BBC News". 

The University of Washington demonstrates, using a variety of clips and only audio from an impersonator, that they can create realistic videos of the former president saying anything they want.

It's possible to sketch out an idea and let AI create a finished product of excellent quality.

The YouTube channel ACMSIGGRAPH has a video titled: "Technical Papers Preview: SIGGRAPH 2018", which is discussed on the webpage: "Could These Be The Next High-Tech Tools That Animators Use Daily?".

What you have asked about was possible long ago and not particularly difficult to do.